Currently, one promising method for separating and recovering a specific gas from a gas mixture for effective utilization of the gas is separation by means of carbon film. As compared with ceramic film and zeolite film, the carbon film, which is generally produced by pyrolyzing polymer material film at high temperature so as to carbonize the film, has excellent heat resistance and is unlikely to provide defects during a film formation step.
One such carbon film heretofore disclosed is produced through carbonization of an aromatic polyimide asymmetric hollow fiber membrane (see, for example, Patent Documents 1 to 3). A method for producing a carbon molecular sieve film is also disclosed in literature (see, for example, Patent Document 4). In the method, a thermosetting resin liquid is applied onto a surface of a ceramic porous body to thereby form a polymer film, and the assembly is heated in a non-oxidizing atmosphere, whereby a molecular sieve film is produced.
In one proposed approach to attain a satisfactory permeation rate and separation factor simultaneously, a carbon film is formed on a porous substrate from an aromatic polyimide resin having a specific structure, serving as a precursor for carbon film (see, for example, Patent Document 5).
Generally, when the aforementioned separation membrane is employed for selective permeation of gas or other substances, permeation rate decreases as the separation factor to be attained increases. Separation factor and permeation rate of carbon film are regulated through selection of decomposition conditions of the precursor resin therefor or through other factors. Hitherto, separation factor and permeation rate of carbon film have not been balanced at a satisfactory level. In contrast, the approach proposed in Patent Document 5 is that a carbon film provided on a porous substrate is produced from an aromatic polyimide resin having a specific structure serving as a precursor for carbon film, so as to attain satisfactory permeation rate and separation factor. The carbon film disclosed in Patent Document 5 is excellent in satisfying high-level permeation rate and separation factor. However, when the thickness of the carbon film is further decreased in order to elevate permeation rate, separation factor may fail to increase.    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 4-11933    Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 4-193334    Patent Document 3: Japanese Patent Application Laid-Open (kokai) No. 5-220360    Patent Document 4: Japanese Patent Application Laid-Open (kokai) No. 10-52629    Patent Document 5: Japanese Patent Application Laid-Open (kokai) No. 2003-286018
The aforementioned carbon film produced from a specific aromatic polyimide resin serving as a precursor per se is an excellent material. However, when the thickness of the carbon film is further reduced, irregularities present on the porous substrate surface on which the carbon film is disposed interact with the thin film, thereby impairing separation factor. In other words, reducing the film thickness results in irregularities present on the porous substrate surface having a height larger than the film thickness. For example, when a protrusion having a height greater than the film thickness is present, a portion of the film corresponding to the protrusion is excessively thinned, possibly generating pinholes and cracks. Such phenomenon may inhibit enhancement of separation factor.
In the above-proposed method, a polyimide resin precursor is applied onto a surface of the porous substrate, the precursor is converted to polyimide, and the polyimide film is carbonized, to thereby form a carbon film. In this case, the polyimide resin precursor which has been applied onto the porous substrate surface enters dented portions and spaces defined by particles forming the porous substrate. Thus, when the total volume of irregularities present in the surface of porous substrate is relatively large with respect to the volume of the polyimide resin precursor applied (i.e., when the particles forming the porous substrate have a larger mean particle size), the areas of the film corresponding to the irregularities may be thinner or accompanied with pinholes and cracks.
Meanwhile, a carbon film laminate is suitably employed for gas separation. In order to effectively separate a specific gas species from a gas mixture, controlling the pore size of the carbon film is essential. However, currently, methodology for controlling the pore size has not yet been fully studied. Hitherto, there has never been produced a carbon film laminate having a pore size which removes VOC—a problematic compound recognized in recent years—from air at high efficiency.